Despite the clinical success of B-Raf inhibitors like vemurafenib, resistance has emerged in treated patients thereby dampening the initial enthusiasm for this approach. Further investigation into resistance mechanisms has provided considerable insights and determined that a major cause results from paradoxical MEK/ERK signaling. Studies have demonstrated that while vemurafenib inhibits B-Raf V600E potently, in the context of WT B-Raf homodimers and heterodimers and activating Ras mutations, it leads to kinase activation of the other partner in the dimer thereby stimulating the downstream pathway rather than inhibiting it. Further efforts are therefore required to complement inhibition of the mutant V600E kinase with other ways of preventing downstream signalling. The Brummer group recently investigated the importance of dimerization for wildtype, oncogenic and drug inhibited B-Raf and showed that conserved residues in the dimer interface (DIF) play an important role for the signaling potential of wildtype B-Raf in cells expressing oncogenic Ras. Furthermore, the formation of B-Raf homo- and heterodimers is based on distinct structural requirements and is essential for Ras/Raf-1 mediated paradoxical MEK/ERK activation by drug-bound B-Raf. These findings identify not only the DIF as potential therapeutic target (in Ras-driven tumors) or to avoid paradoxical MEK/ERK activation, but also lend strong support to an allosteric model of Raf activation by DIF mediated transactivation. The McInnes laboratory has designed peptidic inhibitors of Raf dimerization and has recently initiated a collaboration with the Brummer group in order to demonstrate their potential as chemical biology probes of oncogenic Ras signaling events. After achieving proof of concept of blocking paradoxical activation of MEK/ERK signalling, such inhibitors could then be considered as leads for the development of more drug-like compounds as cancer therapeutics. We propose to synthesize a library of peptidic inhibitors to explore the effects of truncation, substitution and conformational stabilization approaches on the inhibition of dimerization events by the B-Raf DIF peptide sequence. The development of peptidic tools compounds and subsequent cellular experiments will enable to us to achieve the objectives of further understanding how Raf dimerization events contribute to propagation of signals in the Ras/Raf/MEK/ERK pathway while providing the basis for exploiting these in the context of tumors resistant to vemurafenib through paradoxical activation of MEK/ERK.